A novel HTLS thermo-mechanical model: applications to Italian OHTL

The electricity market deregulation and the new renewable sources development entail a need for a non-discriminatory access by generators and consumers to the transmission system. The power flow limits in the transmission lines are fixed by norms and policies with the aim of ensuring a given level of security in the electric system following a steady-state approach; the violation in N-1 scenarios of these limits must be avoided by curative actions that can result in renewable generators curtailments and/or in relevant re-dispatching orders. The main reason of potential transmission lines congestion is due to security limits deriving by the loading patterns that were originally assumed at the design level (before the recent fast growing of renewables). In particular, the most important safety consideration is providing adequate clearances as regards to all objects under the transmission line; consequently, the primary limitation on line ampacity is maintaining the design sags under all operating conditions. Straightforward solutions to this problem include construction of new power transmission lines and/or upgrades to the existing ones. Dynamic Thermal Rating (DTR) of power lines is particularly interesting for Transmission System Operators, because the thermal time constant of conductors is relatively high (more than 10 minutes); in case of temporary grid congestions, DTR allows exploiting the dynamic performances of conductors, i.e. currents much higher than the steady-state limits, instead of re-dispatching generating plants. It must be underlined that, in defining DTR limits to be applied to N-1 security evaluations, in any case the respect of law prescriptions about critical sag, clearance, and electro-magnetic field limits must be guaranteed. So DTR is strictly connected with the possibility of calculating the actual sag of the most critical spans of a transmission line, the power flow and the weather conditions being known. However, the conventional DTR calculation tools do not take into account the non-linear nature of high-temperature low-sag (HTLS) conductors and usually adopt a simplified model (the so-called "ruling-span technique") to analyze multi-span transmission lines. From the literature analysed it arises that the dynamic thermal stress due to line current and weather conditions has been faced in the past taking into account only one span, under the hypothesis that the insulator strings were rigid. It is well known that in a real multi span line with different span lengths armed by insulator strings (I or V) that support the same conductor subjected to temperature variations, longitudinal displacements arise due to different elongation of conductors at each span and, as a consequence, sag elongations differ from those calculated under the hypothesis of rigid insulatorstrings. This fact is more and more evident when the conductors’ temperatures under overloading conditions overcome 100 – 150 °C. In the present paper a simplified non-linear mechanical model for a generic bimetallic conductor is proposed in order to extend the use of DTR tools also to OHTL equipped with HTLS conductors. Such a model is than combined with the "equation of change of state" written in a form that can be applied to multi-span lines. The novel resulting model allows the traction/compression status of the external section of the conductor to be correctly calculated span by span, thus estimating the correct sag of each span. Finally, this thermo-mechanical model is applied to some case studies relevant to HV existing Italian power lines and comments from the TSO’s point of view are provided. The traditional ruling span technique is also used for comparison purposes and the relevant results are discussed.